AFTER-EFFECTS OBSERVED IN 57CO(III)-HEDTA COMPLEX

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AFTER-EFFECTS OBSERVED IN 57CO(III)-HEDTA

COMPLEX

I. Dézsi, Agnes Nagy, D. Nagy

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, De'cembre 1976, page C6-909

AFTER-EFFECTS OBSERVED IN 5

7 ~

~

(

COMPLEX

~

)

-

~

I. DEZSI, AGNES G. NAGY and D. L. NAGY Central Research Institute for Physics Budapest, Hungary

RBsumB. - Des complexes varies du type 57C0 (111)-HEDTA ont 6t6 Btudies pour avoir des informations sur les effets chimiques de la capture electronique. Dans les complexes chimkres cristallis6s le 57Fe est 75 % sous forme Fez+ et 25 % sous forme Fe3+. Dans les solutions gel& seule la forme Fez+ peut etre detectke. La presence d'une grande quantite de fer sous forme Fez+ est en accord avec le modkle d'autoradiolyse des after-effects.

Abstract.

-

Various 57C0 (111)-HEDTA complexes have been studied to get information about the chemical effects of the electron capture. In crystalline dimeric complex 57Fe is stabilized in

75 % Fez+ and 25 % Fe3+ ions the latter forming a dimeric complex again. In frozen solutions only Fez+ ions could be detected. The presence of a large amount of Fez+ supports the autoradiolysis model of the after-effects.

Nuclear decays such as internal conversion, isomer transition and electron capture are usually followed by Auger cascades [l]. The Auger cascade leads to the production of highly ionized states of atoms. If a radioactive atom is built into a molecule, the niolecule may become unstable after the Auger cascade. Charge spectrometry has shown that such molecules became fragmented by Coulombic repulsion and the lifetime of the highly ionized species exceeds 10-S s in the gaseous phase [2]. The stabilization of the highly ionized species may lead to the formation of products which structurally differ from the original molecules. For the investigation of the chemical consequences of these nuclear transformation in solids the Mossbauer effect has proved to be a useful method.

The aim of the present work was to get information about the chemical effect of the electron capture of the CO-57 isotope in a solid organic complex, namely

FIG. 1. - Charge states of Fe produced by the Auger cascade following the electron capture decay of CO-57 [3].

57Co (111)-HEDTA chelate. (HEDTA is N-hydroxy- ethyl-ethylenediamine triacetic-acid). In the Auger cascade scheme for 5 7 ~ 0 2 + the maximum charge

state of the daughter iron was estimated to be

+

7 [3]. The theoretical charge distribution of Fe-57 is shown in figure 1.

Recently, dimeric (Fig. 2) and monomeric forms of Fe (111)-HEDTA have been found to be formed

FIG. 2. - Structural formula of Fe (111) -HEDTA dimer.

depending on the pH values of the solution [4].

These structures were also observed in a frozen solu- tion of Fe (111)-HEDTA by Mossbauer spectros- copy [5]. The dimeric and monomeric forms of Fe (111)-HEDTA could be also prepared in solids and observed by this technique. The Mossbauer parameters of the dimer and monomer Fe (111)- HEDTA are listed in table I. These features of the complexes offer a good opportunity for investigating the chemical effects of electron capture as a function of the environment of the molecule in crystalline solid and frozen solutions.

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Mossbauer parameters of Fe (II1)-HEDTA complex

Quadrupole

splitting Isomer shift

Compound (mm/s) (mm/$

- - -

Fe (111)-HEDTA dimeric form 1.63 0.44 Fe (111)-HEDTA monomeric 0.74 0.44

form

Errors of QS and IS values are f 0.01.

1. Experimental.

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The Co(II1)-HEDTA compound was prepared by the method developed by Schugar et al. [4] with only minor modifications. CoCl, aqueous solution was labelled with carrier-free CO-57 isotope of

-

3 mCi. The Co2+ was oxidized to Co3 + by bromine. The excess bromine was expelled

by heating the solution ; Co(OH), was precipitated by potassium hydroxide. The Co(OH), was separated and HEDTA was added to the precipitate. The mixture was heated till dissolution of the precipitate. The dimeric and monomeric species were formed by changing the pH value of the CO (111)-HEDTA solution under the assumption that Co3 + forms similar

structures as observed for Fe3+. For the experiments with frozen samples glycerol was added to the solutions in order ot avoid the formation of conglomerates. The samples were measured at 110 K. Potassium ferrocyanide 0.8 mg/cm2 57Fe was used as an absorber for all emission experiments and the shifts are all referred thereto.

2. Results and discussion. - From the Mossbauer spectra of crystalline 57Co (111)-HEDTA, the daughter Fe-57 was found in two valency states, both of them presenting a symmetric quadrupole doublet (Fig. 3a, Table 11). One of them was identified as high spin Fe2+. The fraction of this species was

-

0.75. The other presents Mossbauer parameters identical to those of Fe (111)-HEDTA dimer ; this latter was thus considered as the retention, that is, the fraction of daughter Fe-57 which remained in the parent molecule, or was rebuilt into the parent molecule. In the frozen solutions of CO (111)-HEDTA dimer and monomer (Fig. 3b) Fe2+ was observed (Table 11). The

Compound

FIG. 3. - Mossbauer spectra of crystalline CO 011) -HEDTA (a) and of frozen solution of CO 011) -HEDTA monomer (b).

asymmetry of the spectrum in figure 3b may be attributed either to relaxation effects or to an iron ion of which the isomer shift is about

-

0.13 mm/s. In order to simulate the effect of the Auger cascade one sample of the Fe (111)-HEDTA dimer was irradiated by a CO-60 y-source and another in a reactor. The irradiated samples were measured by Mossbauer absorption. No radiation damage was observed in the irradiated Fe (111)-HEDTA after absorbing 100 Mrad. Concerning the sample irradiated in the reactor for 2 hours it is mentioned that the Mossbauer absorption spectrum shows the appearance of the

TABLE I1

Mossbauer parameters of 57Co (111)-HEDTA complex Quadrupole splitting Isomer shift

(mm/s) (mm/s) Fe3+

Fe2+ Fe3+ ~e~ + Fe3 + fraction

- - - -

57Co (111)-HEDTA dimer crystal-

line 2.51

+

0.02 1.15

+

0.04 1.16 f 0.02 0.32 _f 0.04 0.25

57Co (111)-HEDTA monomer fro-

zen solution 2.55

+

0.03 1.15 -1 0.03

-

< 0.05 57Co (111)-HEDTA dimer frozen

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AFTER-EFFECTS OBSERVED IN 57Co/III/-HEDTA COMPLEX C6-911

same Fe-57 species as in the case of the Mossbauer emission spectrum of 57Co (111)-HEDTA even though the Fe (111)-HEDTA sample was exposed to the high y-background, the flux of the fast neutrons, and the high local temperature in the reactor. In consequence the Auger process exerts an extremely great effect on the molecule having the decaying atom. From the fact that retention was not observed in the case of the frozen solution of CO (111)-HEDTA it is apparent that the stabilization of 57Fenf is influenced not only by the immediate environment of the decayed nuclei, that is, by the remaining components of the molecule, but also by the environment of the whole

molecule. This allows us to suppose that almost every Fe-57 leaves its original location in the frozen solution of the chelate.

The appearance of the great amount of FeZ+ ions can be explained by the autoradiolysis model, that is, the final charge state of the Fe-57 is determined by the redox properties of the free radicals. These radicals originate from the autoradiolysis of the environment of decayed nuclei induced by low energy electrons or X-rays emitted during the Auger cascade [6].

To determine whether the retention in crystalline CO (111)-HEDTA is intrinsic or is a result of the molecule recombination needs further study.

References

[l] BERGSTROM, I., <<Alpha, Beta and Gamma-Ray Spectres- [4] SCHUGAR, H., WALLING, C., JONES, R. B. and GRAY, H. B.,

copy D. K . Siegbahn ed., (North-Holland Publ. Ams- J. Am. Chem. Soc. 89 (1967) 3712.